European GNSS (Galileo) Open Service
NAVIGATION
SOLUTIONS
POWERED BY
E U R O P E
signal-in-space
operational status
definition
G a l i l e o o p e n s e rv i c e — s i g n a l - i n - s pa c e o p e r at i o n a l s tat u s d e f i n i t i o n , I s s u e 1 . 1 , J u ly 2016
TERMS OF USE AND DISCLAIMERS
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Note: The name “Galileo” shall mean the global satellite
navigation system established under the EU GNSS
Galileo programme.
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Updates
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i
The European GNSS (Galileo) Open Service SignalIn-Space Operational Status Definition Document
(hereinafter referred to as OS SIS OSD) and the
information contained herein is made available to
the public by the European Union according to terms
and conditions specified thereafter for information,
standardisation, research and development and
commercial purposes for the benefit and the promotion
of the European Global Navigation Satellite Systems
programmes.
ii
G a l i l e o o p e n s e rv i c e — s i g n a l - i n - s pa c e o p e r at i o n a l s tat u s d e f i n i t i o n , I s s u e 1 . 1 , J u ly 2016
Table of Contents
SECTION 1:
Introduction..............................................................................................................vi
1.1Background..................................................................................................1
1.2 Document Scope.......................................................................................1
Annex A:
Reference Documents.....................................................................................14
Annex B:
estimating the age of ephemeris.........................................................16
SECTION 2:
Galileo Open Service SIS Status...............................................................2
2.1 SIS Status..........................................................................................................3
2.2 SIS Status Dissemination................................................................3
2.2.1 Dummy message.................................................................................................3
2.2.2 SIS Status Flags................................................................................................3
2.3 Mapping between SIS Status Flags and
SIS Status..........................................................................................................5
2.3.1 On the meaning of the cyclic redundancy
check (CRC)...............................................................................................................7
SECTION 3:
Navigation Parameters and Issue of Data..................................8
3.1 Usage of Parameters Identified by an
OD Value.............................................................................................................9
3.2 Usage of Parameters Not Identified by
an IOD Value...................................................................................................9
SECTION 4:
Ionospheric Model..............................................................................................10
iii
SECTION 5:
Validity of Navigation Parameters..................................................12
iv
List of Figures
Figure 1.Example of a decision tree for
the determination of the SIS
Status...........................................................................................6
List of Tables
Table 1.Signal Health Status Bit Values..................4
Table 2.Data Validity Status Bit Values.....................4
Table 3.SISA Index values relevant to the
assessment of the SIS Status..........................4
Table 4.Galileo Open Service SIS Status
vs SIS Status flags........................................................5
Table 5.Summary of SIS flags to be
checked by the different Galileo
OS users to compute the Status
of Signals-In-Space......................................................6
G a l i l e o o p e n s e rv i c e — s i g n a l - i n - s pa c e o p e r at i o n a l s tat u s d e f i n i t i o n , I s s u e 1 . 1 , J u ly 2016
Document Change Record
Issue Revision
Date
First version to support Public Consultations on the OS SIS OSD
1
0
September 2015
First Public Issue of the OS SIS OSD
1
1
July 2016
v
Reason for change
SECTION 1:
Introduction
1.1 Background
1.2 Document Scope
Galileo is the European Global Navigation Satellite
System (GNSS), providing a highly accurate and global
positioning service under civilian control. Galileo, and in
general current GNSS, are based on the broadcasting
by satellite of electromagnetic ranging signals in
the L frequency band. The performance achievable
through Galileo in terms of position, velocity and time
determination accuracy depends on the availability of
a number of such signals, uncorrupted and compliant
with the system specifications. In order to enable Galileo
receivers to properly handle each signal, the signal itself,
within the associated Navigation Message, contains
information about its operational status.
This document complements the Galileo Open Service
Signal-In-Space Interface Control Document (OS SIS
ICD, ref. Annex A[1]) by describing the encoding and use
of the European GNSS Signal-In-Space (SIS) Status for
the Galileo Open Service (OS). The SIS Status (i.e. the
operational status of the OS SIS broadcast by each of
the Galileo satellites) is an important parameter whose
value determines the applicability of the Minimum
Performance Level of services (hereinafter the “Minimum
Performance Level” or MPL) as described by the Service
Provider in the Galileo Open Service – Service Definition
Document (OS-SDD), which will be published before the
Galileo OS is declared available.
Because of the inherent complexity of a Global
Navigation Satellite System, the quality of Galileo signals
depends on a wide variety of factors. Nonetheless, the
status of signals has been encoded using a minimal set
of parameters that receivers can extract and decode in
order to determine the status of each signal.
The content of this document is specifically targeted
towards manufacturers of Galileo receivers. The first
part provides a comprehensive guide of how the status
of the Galileo SIS is disseminated by the signal itself
through specific data within the Navigation Message.
In the second part, some considerations about the
use of navigation parameters are presented and the
reference for the ionospheric model used by Galileo is
recalled. Finally, in the last chapter, some considerations
are provided concerning the validity of navigation
parameters.
1
G a l i l e o o p e n s e rv i c e — s i g n a l - i n - s pa c e o p e r at i o n a l s tat u s d e f i n i t i o n , I s s u e 1 . 1 , J u ly 2016
SECTION 2:
Galileo Open Service
SIS Status
G a l i l e o o p e n s e rv i c e — s i g n a l - i n - s pa c e o p e r at i o n a l s tat u s d e f i n i t i o n , I s s u e 1 . 1 , J u ly 2016
2.1 SIS Status
Users of the Galileo Open Service (OS) can obtain
information about the Signal-In-Space (SIS) Status, i.e.
about the operational status of the OS SIS 1 broadcast
by each one of the Galileo satellites, through the signals
themselves.
Annex A[1]). As detailed there, the complete navigation
message data is transmitted on each data component as
a sequence of frames. A frame is composed of several
sub-frames, and a sub-frame in turn is composed of
several pages.
The Galileo SIS Status can assume one of the following
three values:
In the context of this document, by "navigation message"
a sub-frame is meant (50 seconds for F/NAV and 30
seconds for I/NAV) that contains ephemeris and clock
corrections or SIS health information, i.e. Page Type 1
to 4 for the F/NAV and Word Type 1 to 6 for the I/NAV.
●● Healthy,
●● Unhealthy,
●● Marginal.
●● SIS “Healthy”: The SIS is expected to meet the
Minimum Performance Level 2.
●● SIS “Unhealthy”: The SIS is out of service or under
test.
●● SIS “Marginal”: The SIS is in neither of the two
previous statuses.
The Galileo OS Minimum Performance Levels reported
in the future Galileo OS SDD will refer exclusively to
Healthy SIS. No MPL is defined for Unhealthy or Marginal
SIS.
2.2 SIS Status
Dissemination
The status of each Galileo signal involved in the provision
of OS is determined by the SIS Status information
embedded in the navigation message and by a specific
type of message that may replace the standard one: the
so called dummy message (see section 2.2.1).
2.2.1 Dummy message
When no valid navigation data is to be transmitted by a
satellite, dummy pages are generated and downlinked.
Dummy pages are defined in the Galileo OS SIS ICD
(Annex A[1]) (Page/Word Types 63 for F/NAV and I/NAV,
respectively). Such dummy pages, if transmitted, replace
the whole navigation message which is then defined as
a dummy navigation message.
A dummy navigation message indicates Unhealthy
SIS. Therefore, as soon as a dummy page, i.e. Page/
Word Types 63, is decoded, the respective SIS must be
considered Unhealthy.
Once the transmission of the dummy message
terminates and normal transmission is recovered, users
must follow the appropriate procedure to check the SIS
Status, as described in the next sections, before starting
to use the SIS again.
Note that in the case of Dual Frequency (DF) users, a
dummy navigation message on either of the frequencies
implies that the DF service is not Healthy (see also
section 2.3) 3.
2.2.2 SIS Status Flags
The purpose, structure and content of the Galileo
navigation message are explained in the Galileo Open
Service Interface Control Document (OS SIS ICD,
The status of the Signal-In-Space is encoded within the
navigation message through three SIS Status Flags (see
1�������The Galileo OS SIS comprises signals broadcast on the E1B, E5a, and E5b
carriers (see the OS SIS ICD, for more details).
2�������Note that this assertion does not apply to the Minimum Performance
Level for the provision of the GPS-Galileo System Time Offset (GGTO).
Healthy SIS can broadcast parameters that indicate a non-valid GGTO
(ref. Annex A[1] for more details about the dummy value of the GGTO
parameters).
3�������The navigation message, specifically the I/NAV, transmitted on E1 and
E5b, has been designed to include another type of page: the Alert Pages
(see OS SIS ICD, (Annex A[1]) for a detailed definition). The transmission
of Alert Pages is a capability that the Galileo system is currently not
exploiting.
3
The meaning of the three possible values of the SIS
Status is the following:
4
Annex A[1] for the complete definitions of the different
flags):
●● The Signal Health Status (SHS) flag.
●● The Data Validity Status (DVS) flag.
Data Validity
Status (DVS)
Definition
0
Navigation data valid (NDV)
1
Working without guarantee (WWG)
Table 2.
Data Validity Status Bit Values
●● The Signal-In-Space Accuracy (SISA) value.
The mapping of the SIS Status with the SIS Status flags
is provided in section 2.3.
Signal Health Status (SHS)
The SHS flag is contained in the navigation data and the
almanac messages of each Galileo OS signal (E1, E5a or
E5b). For E1 and E5b, the SHS flags are included in the
I/NAV navigation data stream. For E5a, the SHS flag is
included in the F/NAV navigation data stream.
The SHS flag to be used is the one broadcast in the
navigation data transmitted by the satellite whose SIS
is being used. The SHS flags broadcast as part of the
almanac data are provided for convenience in support of
satellite acquisition but should not be used operationally
to determine the SIS Status.
The SHS flag can be raised by the system at any
moment 4. The SHS flags can assume the values defined
in the OS SIS ICD (Annex A[1]), and recalled for
convenience in Table 1.
Signal Health
Status (SHS)
Definition
0
Signal OK
1
Signal out of service
2
Signal will be out of service
3
Signal component currently in Test
Table 1.
Signal Health Status Bit Values
Data Validity Status (DVS)
For each Galileo OS signal (E1, E5a or E5b), the DVS flag
is contained in the navigation data. For E1 and E5b, the
DVS flags are included in the I/NAV navigation data
stream. For E5a, the DVS flag is included in the F/NAV
navigation data stream. The flag can assume the values
defined in the OS SIS ICD, (Annex A[1]), which are recalled
for convenience in Table 2.
4�������The Time To Alert between the occurrence of an event and the setting of
the SHS is under definition. When available, the related performance will
be provided through the OS SDD.
The DVS flag can be triggered in two ways:
1) When the time passed since the last navigation
message uplink to the satellite is greater than a
predefined threshold.
2) By the satellite in case of certain anomalies.
In the first case, the DVS acts as a timer accounting for
the age of the navigation message. The time interval
threshold for its triggering is set by the system to ensure
the service performance.
In the second case, as the DVS is triggered on-board in
case of specific events, meaning that the DVS flag value
can change at any time 5.
Signal-in-Space Accuracy (SISA)
For E1 and E5b, the SISA Index is contained in the I/
NAV navigation data stream. For E5a, the SISA Index is
contained in the F/NAV navigation data stream.
As described in the OS SIS ICD, (Annex A[1]), the SISA
parameter can assume 255 values. Nonetheless, when
the SISA is used as one of the means for determining the
SIS Status, it must be considered as a binary indicator,
with its only meaningful values being “No Accuracy
Prediction Available” (that is NAPA, when SISA=255) or
“not NAPA” (when SISA≠255). Note that the SISA values
described in the OS SIS ICD (Annex A[1]) as Spare (from
126 to 254) are to be considered as not NAPA.
The SISA Index values relevant to the assessment of the
SIS Status, as defined in the OS SIS ICD, (Annex A[1]), are
recalled for convenience in Table 3.
SISA Index
255
0......254
SISA Value
No Accuracy Prediction Available (NAPA)
not NAPA
Table 3. SISA Index values relevant to the assessment of
the SIS Status
5�������The Time To Alert between the occurrence of an event and the setting of
the DVS is under definition. When available, the related performance will
be provided through the OS SDD.
G a l i l e o o p e n s e rv i c e — s i g n a l - i n - s pa c e o p e r at i o n a l s tat u s d e f i n i t i o n , I s s u e 1 . 1 , J u ly 2016
2.3 Mapping
between SIS
Status Flags
and SIS Status
The mapping between the values of the SIS Status Flags
and the three values of the SIS Status is provided in
Table 4.
SIS Status
Healthy
Unhealthy
Marginal
Dummy
Message
SIS Flags
SHS
DVS
SISA
No
OK
NDV
not NAPA
No
Out of Service
Any Value
Any Value
No
In Test
Any Value
Any Value
Yes
N/A
N/A
N/A
No
Ok
WWG
Any Value
No
Ok
Any Value
NAPA
No
Will be out of Service
Any Value
Any Value
Table 4. Galileo Open Service SIS Status vs SIS Status flags
The first condition to be fulfilled by the navigation
message in order for the user to proceed with the
determination of the SIS Status is that it cannot be a
dummy message. As explained in section 2.2.1, if it is a
dummy message the respective SIS must be considered
Unhealthy.
If the message is not dummy, the user can proceed
with checking the other conditions to determine the SIS
Status as specified in Table 4. The order in which the flags
are checked by a receiver on a not dummy navigation
message is arbitrary and up to the manufacturer. The
value of each flag can be read (and taken into account)
independently from the value of the other flags. An
example of a possible decision tree for the determination
of the SIS Status is provided in Figure 1.
As an example of receiver operations, in section 2.2.2 it
is explained that the DVS flag could be set to WWG in
case the respective SIS cannot be considered Healthy
but, depending on the value of the other flags, only
Marginal. This means that neither the SIS nor any of the
parameters obtained through such SIS should be used.
As soon as receivers detect that the DVS value has been
set back to 0 and that, taking into account the other
relevant flags, the SIS Status is Healthy, they will have
to retrieve the most recent navigation data set newly
broadcast in order to use such SIS.
In other words, all the parameters requested by a
receiver have to be retrieved once the SIS is (back to)
Healthy in order to ensure that the MPL is expected to
be met.
Note also that the value of SIS Status flags applies to
the whole subframe they belong to. As an example, the
DVS flag in the I/NAV message is located in Word Type
5 but its value is applicable also to the content of Word
Type 1, 2, 3 and 4.
Depending on the service to be used, Single Frequency
(SF) or Dual Frequency (DF), users will have to check
the value of the various flags on different SIS. Table 5
shows the summary of the SIS Status Flags to be
checked by the different Galileo OS users for both SF
and DF services
5
In order to determine the Status of a specific SIS
broadcast by a Galileo satellite, the user will have to
ensure that the navigation message has been properly
received, i.e. that it successfully passes the CRC check
(see section 2.3.1).
6
Dummy
Message
Yes
No
Will be Out
of Service
Out of Service
or Test
SHS
Ok
WWG
DVS
NDV
NAPA
SISA
Not NAPA
Healthy
Marginal
Example of a decision tree for the determination of the SIS Status
E5a
Χ
Χ
E1/E5b
Χ
Χ
Χ
Χ
E5b
E1/E5a
SISA(E1,E5b)
Χ
E5bDVS
Χ
E5bHS
E1BDVS
E1
E1BHS
Single
Frequency
Double
Frequency
Table 5.
I/NAV
SISA(E1,E5a)
F/NAV
E5aHS
Users
E5aDVS
Figure 1.
Unhealthy
Χ
Χ
Χ
Χ
Χ
Χ
Χ
Χ
Χ
Χ
Χ
Summary of SIS flags to be checked by the different Galileo OS users to compute
the Status of Signals-In-Space
G a l i l e o o p e n s e rv i c e — s i g n a l - i n - s pa c e o p e r at i o n a l s tat u s d e f i n i t i o n , I s s u e 1 . 1 , J u ly 2016
2.3.1 On the meaning of the
cyclic redundancy check
(CRC)
The CRC is not used to indicate any problem at satellite
level, i.e. on transmitter side. The CRC checksum is
exclusively related to the errorless reception of the
transmitted bits, i.e. to the transmission channel, not to
the correctness of the structure or the contents of the
message as transmitted by the Galileo system. The CRC
available within the navigation message of Galileo is
therefore not involved in the definition of the SIS Status.
If the CRC checksum is not passed successfully, the
respective data is rejected and no determination of the
SIS Status is possible. Once a navigation message is
finally received with a successful CRC, the user can then
proceed to the SIS Status determination, as described in
the previous section, and eventually to the utilisation of
such SIS.
7
To detect the corruption of the received data a checksum
is used by the Galileo navigation message, which
employs a CRC technique. The detailed description of
this checksum is provided in the OS SIS ICD (Annex A[1]).
SECTION 3:
Navigation Parameters
and Issue of Data
G a l i l e o o p e n s e rv i c e — s i g n a l - i n - s pa c e o p e r at i o n a l s tat u s d e f i n i t i o n , I s s u e 1 . 1 , J u ly 2016
As defined in the OS SIS ICD, Annex A[1], the navigation
parameters are disseminated in data batches, each
identified by an Issue of Data (IOD). The identification of
each batch by an IOD value enables:
●● the users to distinguish the data in different
batches received from each satellite;
●● to indicate to the user’s receiver the validity of the
data (which have to be updated using new issue of
navigation data);
●● the user receiver to compute the full batch of data
even if it misses some pages or starts receiving
the data somewhere during the transmission.
Values from a single satellite is not recommended and
the resulting performance is not expected to meet the
Minimum Performance Level.
Galileo satellites are not expected to all transmit the
same IOD. For positioning, users can combine SIS from
different satellites with different IOD Values provided
that the navigation parameters derived from each
satellite are indeed tagged by a unique IOD Value.
It is relevant to note that IOD Values are not necessarily
incremented in steps of one and that an IOD with higher
value does not necessarily mean that it tags more recent
data. The only valid comparison between IOD Values is
whether they are equal or not.
Two independent IODs are defined for:
●● the almanacs.
Another set of navigation data, the broadcast group
delay (BGD), ionospheric corrections, GST-UTC and
GST-GPS Time conversion parameters, navigation data
validity status (DVS) and signal health status (SHS), is
not identified by any Issue of Data.
3.1 Usage of
Parameters
Identified by an
IOD Value
To compute position and clock corrections, receivers
must use, for each satellite, IOD-tagged parameters
corresponding to the same IOD Value. These parameters
must be retrieved from the most recent navigation data
set broadcast on a Healthy SIS by the Galileo system
after the start of the current receiver operation. These are
the two conditions under which the navigation solution
is expected to meet the Minimum Performance Level.
The utilisation of parameters identified by different IOD
3.2 Usage of
Parameters Not
Identified by an
IOD Value
As mentioned in section 3, the BGD, ionospheric corrections,
GST-UTC and GST-GPS conversion parameters, DVS and
SHS are not identified by an IOD Value.
As is the case for IOD-tagged parameters, to compute
the navigation solution, receivers must retrieve the
values of not IOD-tagged parameters relevant to the
type of navigation solution to be computed 6 from the
most recent navigation data set broadcast on a Healthy
SIS by the Galileo system after the start of the current
receiver operation. This is the condition under which the
navigation solution is expected to meet the Minimum
Performance Level.
The satellite specific parameters (BGD, DVS, SHS) have to
be obtained from each satellite and for each frequency
while system parameters (ionospheric corrections,
GST-UTC and GST-GPS conversion parameters) can be
obtained from any Healthy SIS.
9
●● the ephemeris, satellite clock correction parameters
and SISA;
6�������As an example, DF users do not need to retrieve ionospheric parameters
SECTION 4:
Ionospheric Model
G a l i l e o o p e n s e rv i c e — s i g n a l - i n - s pa c e o p e r at i o n a l s tat u s d e f i n i t i o n , I s s u e 1 . 1 , J u ly 2016
11
The ionospheric delay is usually the largest contributor
to the pseudorange error for single-frequency users.
This delay can be partially mitigated using ionospheric
models such as Klobuchar or NeQuick. The ionospheric
model adopted by Galileo is the NeQuick model (Annex
A[2]). The Galileo navigation message includes several
parameters that are used by receivers implementing the
NeQuick model to compute the ionospheric effect.
SECTION 5:
Validity of
Navigation
Parameters
G a l i l e o o p e n s e rv i c e — s i g n a l - i n - s pa c e o p e r at i o n a l s tat u s d e f i n i t i o n , I s s u e 1 . 1 , J u ly 2016
Navigation parameters shall not be used beyond the
maximum possible broadcast period of a healthy
navigation message data set, currently set to 3 hours 7.
A navigation message data set may be superseded
before the expiration of these 3 hours by the broadcast
of a new navigation message data set.
As previously stated, the Galileo OS Minimum
Performance Levels are expected to be met only if the
navigation solution is computed by a receiver using
navigation parameters retrieved from the most recent
navigation data set broadcast by the Galileo system
after the start of the current receiver operation. The
7�������This time interval might be modified in the future. A procedure is
described in Annex B of this document to estimate the Age of Ephemeris
as prediction time from reference Time of Ephemeris (t-toe). Applying
this algorithm, users are able to compare the age of the navigation
parameters they have available with the maximum possible broadcast
period of a healthy navigation message data set.
utilisation by a receiver of navigation parameters stored
during previous receiver operations implies that the
navigation performance that can be achieved may not
meet the Minimum Performance Level. Furthermore,
given the way the DVS and SHS flags can be set (section
2.2.2), a navigation data set within the maximum possible
broadcast period of a healthy navigation message data
set but not retrieved from the currently broadcast SIS
might lead to the use of non-Healthy signals: the use of
an old navigation data set stored in the memory of the
receiver instead of the most recent data set transmitted
by Galileo is performed by users at their own risk.
Concerning the GST-UTC conversion parameters, in the
current system configuration they are updated daily
(although this may change in a future configuration).
With respect to the almanacs, their refresh rate is the
same as for the navigation data
13
The navigation dataset refresh rate is a parameter
defined by the system. The typical refresh rate of the
navigation data set through upload from Galileo Ground
Segment to satellites can range from a minimum of 10
minutes to about 3 hours.
1.1.1.1.
Signal
Modulation
3
Scheme
=
,
31,
,
(1−
100
) +
3 2,
,
100
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For this purpose, the Total
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in a cross-section of 1m2,
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is expressed in TEC
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Annex A: C Φ Φ
Reference
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The ionosphere is classically
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sub-divided in layers character݀ாହ௔ିூǡሾ௜ሿವ಴ಶఱೌష಺
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E, F1 and F2, the latter being
௜ୀିஶ
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tions.
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ሺ‫ݐ‬ሻ ൌ ෍ ቂܿாହ௕ି
”‡…– ்಴ǡಶఱ್
”‡…– ்಴ǡಶఱ್ష಺
൫‫ ݐ‬െ ݅ܶ஼ ǡாହ௕ିொ
షೂ
൯ቃ
G a l i l e o o p e n s e rv i c e — s i g n a l - i n - s pa c e o p e r at i o n a l s tat u s d e f i n i t i o n , I s s u e 1 . 1 , J u ly 2016
Title
issue
[1]
European GNSS (Galileo) Open Service Signal-In-Space Interface
Control Document (OS SIS ICD).
Issue 1.2, European Union, 2015.
[2]
Ionospheric Correction Algorithm for Galileo Single Frequency Users.
Issue 1.1, European Union, 2015.
15
Reference
Annex B:
estimating the
age of ephemeris
G a l i l e o o p e n s e rv i c e — s i g n a l - i n - s pa c e o p e r at i o n a l s tat u s d e f i n i t i o n , I s s u e 1 . 1 , J u ly 2016
tk = t ― toe
where:
t
is the Time of Reception of the
message in GST time [s] based
on the broadcast Time Of Week
(TOW), defined as the number of
seconds that have occurred since
the transition from the previous
week, ranging from 0 to 604799
seconds and reset to zero at the
end of each week (00:00 Sunday).
toe
is the Time of Ephemeris in GST
time [s] (as for t, ranging from 0 to
604799 seconds), as broadcast by
the navigation message.
If a receiver wishes to check whether the Age of
Ephemeris of a navigation data set is within a certain
validity time VT it will have to evaluate the following
inequality:
0 ≤ tk ≤ VT
Note that, as explained in Annex A[1] (see Note to Table
58), tk is the actual total time difference between the
broadcast GST and the Time of Ephemeris, accounting
for the possible beginning or end of week crossover.
To take this into account, if tk is greater than 302400
seconds, 604800 seconds must be subtracted from tk. If
tk is less than -302400 seconds, 604800 seconds must
be added to tk.
17
The Age of Ephemeris tk is defined as follows (see also
Annex A[1]):